1. Field of the Invention
The invention relates to an image reading apparatus for reading an image while conveying a sheet material.
2. Description of Related Art
Usually an image reading apparatus of a type which reads the image surface of a sheet material while conveying the sheet material uses a close contact type image sensor as reading means, from the viewpoints of space and cost.
However, a lens used in the close contact type image sensor has a depth of field as small as 0.2-0.3 mm. Therefore, in a case where a sheet material cannot be held in the depth of field, that is, in a case where the sheet material cannot be brought into close contact with a focusing position (image reading surface) provided on a reading glass surface, blur occurs to a read image.
So, generally, as in an image reading apparatus 140 shown in FIG. 7 of the accompanying drawings, a transparent image reading surface 145 is attached to a reading portion housing 141, and a light source 144, a lens 143 and a light receiving element 142 are provided in the interior of the housing. When an image surface on the upper surface side of a sheet material S is to be read, a platen roller 246 mounted on the fore end of an arm member 148 is biased in the direction indicated by the arrow by a tension spring 147 and the image surface of the sheet material S is urged against the image reading surface 145. Thereby, the sheet material S is kept so as not to separate from the image reading surface 145 by a prescribed amount or greater.
Also, in Japanese Patent Application Laid-open No. H10-190938, as shown in FIG. 8 of the accompanying drawings, driven rollers 131 and 132 are provided on the upstream side and the downstream side, respectively, of a reading portion housing 141, and these are pressed against conveying rollers 121 and 122, respectively, by compression springs 170. Thereby, a sheet material S is brought into close contact with a sheet conveying guide 11 so as to bring the image surface of the sheet material S into close contact with an image reading surface.
Also, in Japanese Patent Application Laid-open No. 2000-115452, as shown in FIG. 9 of the accompanying drawings, a crooked conveying path P for a sheet material S is constituted by two reading portion housings 141. Thereby, the sheet material S conveyed through the conveying path P may be normally brought into close contact with image reading surfaces 145.
Now, there is a desire to well read, by an image reading apparatus, a laminate-processed, rigid card having a thickness of 0.3 mm or greater such as a driver's license heretofore read by a flat bed scanner.
However, in the above-described image reading apparatus constructed on the premise that the sheet material is thin paper, a sheet material having a great thickness is conveyed with a rush while widening the gap between an image reading surface and a platen roller or an original plate and may therefore injure the image reading surface. If an injury occurs to the image reading surface, a streak will occur to a read image.
Likewise, in the image reading apparatus 140 shown in FIG. 8, the sheet material S widens the gap between a sheet conveying surface 103 and an image reading surface 145 and therefore, an injury occurs to the image reading surface 145 and a streak occurs to a read image. Further, provision is made of an upstream roller pair (a conveying roller 121 and a driven roller 131) and a downstream roller pair (a conveying roller 122 and a driven roller 132) and therefore, when the sheet material S is engaged with one roller pair alone and when the sheet material S is engaged with both roller pairs, the posture of the reading portion housing 141 changes. Therefore, blur occurs to the read image.
Also, in the image reading apparatus shown in FIG. 9, the conveying path P is crooked and therefore, if the sheet material S is thick or high in rigidity, this sheet material is not flexed and may therefore be caught by the conveying path to thereby cause jam (sheet jamming).
As a construction which does not injure the reading surface, as shown in FIG. 10 of the accompanying drawings, there is also a method whereby conveying guide surfaces 141d and 141e provided before and behind an image reading surface 145 are a little protruded downwardly from the image reading surface 145.
In this method, however, if the sheet material S is a hard card or the like, the abrasion of the conveying guide surfaces 141d and 141e will be promoted. Also, if the sheet material S is a thin sheet material, jam will occur.